PHYSICS OF THE EIGHTEENTH CENTURY. 281 



was a French refugee who had left his country on the revocation of 

 the Edict of Nantes, and had established himself in Spitalfields as a 

 silk-weaver, where John was brought up to the same vocation ; but 

 having a studious turn, young Dollond contrived to make himself 

 acquainted with mathematics, astronomy, and optics. John Dollond's 

 eldest son, named Peter, was in his turn trained to the business of 

 silk-weaver ; but Peter also had imbibed a taste for scientific pursuits, 

 and resolved to enter upon the business of an optician, counting upon 

 his father's mathematical knowledge. The undertaking proved a 

 success, and John Dollond joined his son in the optical business in 

 1752, and soon became very proficient in matters pertaining to the 

 practical part of the art. His first achievement was an improvement 

 in telescopes, by using five lenses in the construction of their eye- 

 pieces. Improvements in other optical instruments followed, and 

 Dollond soon became known to men of science in London. Several 

 of Dollond's papers had been published in the "Philosophical Trans- 

 actions " when he entered upon the discussion with Euler to which 

 reference has already been made. Sir Isaac Newton had declared 

 that the dispersion of the coloured rays by refraction had always the 

 same proportion to the mean deviation of the rays, whatever might be 

 the nature of the. refracting substance. It was this doctrine which 

 Dollond was maintaining (as against Euler) when he argued that if 

 the fact be as stated by Newton, there could be no refraction without 

 dispersed colour : dispersion and refraction being always proportional, 

 all hope of constructing achromatic lenses was proved to be unten- 

 able. 



In his reply Euler attempted to show that the assumption of an in- 

 variable relation could not be maintained; but Dollond was apparently 

 not convinced by Euler's calculations, for it was not until some years 

 afterwards, when a paper by a Swedish mathematician was read before 

 the French Academy of Sciences, giving a geometrical demonstration 

 of the inadmissibility of the law assumed by Dollond, that the English 

 optician acknowledged that this principle, which Newton's experiments 

 appeared to have established, might, after all, be inaccurate. It was 

 perhaps the great authority of Newton's name which had made it pos- 

 sible for this dispute to be carried on so long without any person veri- 

 fying experimentally the grounds upon which Newton's conclusions 

 were based. Dollond at length thought that the point would be settled 

 by a resort to the fountain-head of science, a direct interrogation of 

 Nature, to use the Baconian phrase. Here we may briefly indicate 

 the point in dispute. When a ray of light has traversed a prism, it is, 

 as we have seen, not simply refracted in a single ray, but into a group 

 of coloured rays, spreading out among themselves like the ribs of an 

 extended fan. All the coloured rays are inclined (the red least, the 

 violet most) in their direction to the course of the original ray. If we 

 select a ray, intermediate between the red and violet, for instance, 



